During the past twenty years, research work has been performed on alternative gas storage technologies based on gas-solid adsorption. There are many potential applications of this technology, such as on-board vehicle natural gas storage, on-board vehicle hydrogen gas storage, and oxygen storage for medical and aerospace applications. The advantage of this technology is the low or medium gas pressure requirement, therefore, reducing the high pressure compression cost, avoiding high pressure hazards, and making gas storage easier to handle. However, the successful application of the adsorption based gas storage technology has been halted by the lack of high performance adsorbent materials.
The materials which are suitable for gas storage applications must possess large amounts of pore surfaces, primarily micropore (pore diameter of less than 2 mm) and mesopore (pore diameter of 2 to 50 mm) surfaces. When contacted to gases, a large amount of gas molecules can be adsorbed on these pore surfaces. More gas molecules will be adsorbed with higher gas pressure, while gas molecules will leave the pore surface (desorption) when gas pressure is reduced. Therefore, in most cases, gas adsorption and desorption are reversible processes, making them suitable for gas storage applications.
When evaluating the gas storage performance of an adsorbent, two criteria are used; namely, the equilibrium adsorption capacities and the dynamic adsorption/desorption properties. The equilibrium adsorption capacities are quantified by the gravimetric adsorption (weight of gas adsorbed/unit weight of adsorbent) and the volumetric adsorption capacity (weight of gas adsorbed/unit volume of adsorbent). The dynamic adsorption/desorption properties include the adsorption/desorption rate, the adsorption/desorption recycleability, and adsorption/desorption hysteresis.
The most researched gas adsorbents for gas storage applications are high surface area activated carbons and zeolites. Activated carbons are made from carbonaceous materials such as coal pitch, coconut shells, and petroleum wastes. Activated carbons possess large amounts of micropores and mesopores as well as macropores (with pore diameter larger than 50 mm). The surface areas of activated carbon range from hundred to few thousand square meters per gram. The gravimetric gas adsorption capacities of activated carbon are the highest among different adsorbents, and they usually have excellent dynamic adsorption/desorption properties. However, the bulk densities of activated carbons are usually very low, ranging from 0.1 to 0.7 gram/cc, and the higher surface area of the activated carbon usually results in lower bulk density. The low bulk density nature of activated carbons means the adsorbents have relatively low volumetric gas storage capacities.
Zeolites are porous crystalline aluminosilicates. The zeolite framework consists of an assemblage of SiO.sub.4 and AlO.sub.4 tetrahedral molecular structures joined together in various regular arrangements through shared oxygen atoms, to form an open crystal lattice containing pores of molecular dimensions into which gas molecules can be adsorbed. By pressing into pellets or particles with the help of binding materials, zeolites have relatively high bulk density, ranging from 0.5 to 1.5 gram/cc. However, the gravimetric adsorption capacities of the zeolites are relatively low, and the dynamic adsorption/desorption capacities of the zeolites are not as good as those of activated carbon. A lot of zeolites exhibit large desorption hysteresis which makes adsorption/desorption not completely reversible.